Switching Logic (Rs232C. Modem); Baud Rate Generator (1.8432Mhz) - Tandy 1400LT Service Manual

The UART (IC35) contains a programmable Baud Rate Generator.
The output frequency of the Baud Rate Generator is 16x the baud rate [ divisor
#
=
(frequency input)/(baud rate
X 16)]. Two 8 bit latches store the divisor in a 16 bit binary format. These Divisior latches must be loaded during
initialization in order to ensure desired operation of the Baud Rate Generator.
Table 4-10 shows the use of the Baud Rate Generator with crystal frequency of 1.8432 MHz .
m
m
Percent En or Difference
Between Desired and
Actual
Divisor Used to
Generate 16
X
Clock
Desired Baud Rate
SELECTOR
Hr
B
,
t i
M O D m
50
75
110
134.5
150
300
600
1200
1800
2000
2400
3 600
4800
7200
9600
2304
1536
1047
857
768
3 84
192
96
64
58
48
32
24
16
12
-
0.026
0.058
-
0.69
Table 4-10. Baud Rate Generator (1.8432 MHz)
TO RS232C CONNECTOR / TO MODEM CONNECTOR SWITCHING CIRCUIT
The switching circuit is generated in the GATE3. M O D / m signal is selectable by the software. Figure 4-40 shows
the block diagram of GATE3 (Switching logic)
GATE3
RTS
l 2
A
TXD
l 2
A
SELECTOR DTR
l 2
A
TXDM
m
DO
-
D7
$1
DE
C 0 DER
1/0
PORT
Figure 4-40. Switching Logic (RS232C, MODEM)
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